Transformer bonding question

[73hemicuda]

I'm installing a new 240D/120CT to 480D 3 phase transformer. I have the ground from the primary side feed, the load side and the one to building steel on the lug bolted to the transformer enclosure. If I install the bonding wire from x6 to the enclosure lug, wouldn't this technically be bonded in two different locations since the ground from the primary side is already bonded at the main service entrance where it enters the building? Or is this correct?

 

[ActionDave]

I'm installing a new 240D/120CT to 480D 3 phase transformer. I have the ground from the primary side feed, the load side and the one to building steel on the lug bolted to the transformer enclosure. If I install the bonding wire from x6 to the enclosure lug, wouldn't this technically be bonded in two different locations since the ground from the primary side is already bonded at the main service entrance where it enters the building? Or is this correct?

Welcome to the forum.

I am having a hard time understanding your post. Are you wiring a step up or a step down transformer?

 

[MAC702]

I'm guessing the 240D/120CT part means 240 Delta / 120 Center Tap, so this would have to be the secondary on a step-down xformer, right? But it doesn't seem to match the order presented.

 

[infinity]

The way the OP is written (240D/120CT to 480D) I assumed step up.

As Dave stated we need more information.

 

[73hemicuda]

I'm guessing the 240D/120CT part means 240 Delta / 120 Center Tap, so this would have to be the secondary on a step-down xformer, right? But it doesn't seem to match the order presented.

It is a step up. Sorry I forgot to include that

 

[LarryFine]

It's important that there be NO neutral connection to the primary of the transformer. It should be connected as a purely Delta load.

 

[winnie]

You are using this transformer as a step up transformer, to go from 240V delta to 480V delta.

However the transformer _may_ have been designed as a step down transformer.

Typically the coil center tap will be on the secondary so that you can ground the newly derived source. If you are using the transformer 'in reverse' then this tap will be on the primary side.

As you correctly surmise, you do _not_ want to ground the neutral or center tap on the _primary_ side of your transformer, because this will cause problems.

You _do_ want to ground the center tap on the _secondary_ side of your transformer, unless you are intentionally corner grounding or leaving the system ungrounded.

So: is X6 on the primary or secondary coils as you are actually using this transformer?

-Jon

 

[electrofelon]

It's important that there be NO neutral connection to the primary of the transformer. It should be connected as a purely Delta load.

You _do_ want to ground the center tap on the _secondary_ side of your transformer, unless you are intentionally corner grounding or leaving the system ungrounded.

So: is X6 on the primary or secondary coils as you are actually using this transformer?

-Jon

I certainly agree it is not needed, but as an academic question hat happens if one were to connect the center tap to the supply neutral? I am well familiar with the case of the Delta-Wye transformer being reverse fed and supplied with a neutral, but this is different.

 

[73hemicuda]

You are using this transformer as a step up transformer, to go from 240V delta to 480V delta.

However the transformer _may_ have been designed as a step down transformer.

Typically the coil center tap will be on the secondary so that you can ground the newly derived source. If you are using the transformer 'in reverse' then this tap will be on the primary side.

As you correctly surmise, you do _not_ want to ground the neutral or center tap on the _primary_ side of your transformer, because this will cause problems.

You _do_ want to ground the center tap on the _secondary_ side of your transformer, unless you are intentionally corner grounding or leaving the system ungrounded.

So: is X6 on the primary or secondary coils as you are actually using this transformer?

-Jon

X6 is on the primary side

 

[jumper]

What is make and model? Link?

Would be easier if we could see the connection schematic/nameplate.

 

[LarryFine]

I certainly agree it is not needed, but as an academic question hat happens if one were to connect the center tap to the supply neutral? I am well familiar with the case of the Delta-Wye transformer being reverse fed and supplied with a neutral, but this is different.

Well, to start with, to do it at all, you would have to make certain that the source neutral is derived from the center tap of that transformer, and not from a wye, and then that you connect the source high leg to the new transformer's high leg.

Even given that has been done, I would look at it this way: forget the high leg, and suppose you're feeding a 240v center-tapped (or a 120/240 wired as 240v) transformer from a 240v center-tapped transformer. Would you interconnect the taps?

I wouldn't. I'd be concerned about less-than-perfectly-matching impedances and load currents creating currents within the transformers. In the same way that I wouldn't suggest using multiple transformers in parallel to supply a large load.

 

[73hemicuda]

Hevi-duty Emerson. Catalog# E5H75S. I'd post a picture but I'm not sure how to do it from my phone

 

[73hemicuda]

 

[jumper]

Hevi-duty Emerson. Catalog# E5H75S. I'd post a picture but I'm not sure how to do it from my phone

Okay. Here is the specs.

https://www.emerson.com/documents/a...general-purpose-transformers-en-us-175176.pdf

 

[MTW]

It is a step up. Sorry I forgot to include that

Actually it's a step down, wired in reverse. According to the catalog number

X6 need not be grounded The high side output coil should be bonded though. or left ungrounded, needs ground fault monitoring.

Not shown in the catalog data is, if it's approved for reverse connection.

E85H75S would have been the better model for step up use with its 480/277 Wye output.
Would have allowed for center point bonding of the output coil.

 

[Cow]

Is there a reason you're using red instead of orange for your high leg?

 

[MAC702]

Is there a reason you're using red instead of orange for your high leg?

If you are referring to a "high leg" of a center-tapped Delta, there isn't one, is there?

But it comes from one? I see the question now.

 

[GoldDigger]

I certainly agree it is not needed, but as an academic question hat happens if one were to connect the center tap to the supply neutral? I am well familiar with the case of the Delta-Wye transformer being reverse fed and supplied with a neutral, but this is different.

When you connect a center tapped transformer primary to a center tapped source secondary, you are in effect connecting a 1:1 autotransformer between L1-N and N-L2.
If for any reason (mismatched taps, load related voltage drop....) the two source voltages do not match, you will cause a high current in your primary.

Sent from my XT1585 using Tapatalk

 

[73hemicuda]

Is there a reason you're using red instead of orange for your high leg?

I changed it after the picture was taken. The wires were already marked black, red, blue from previous cutoffs from a different wire pull earlier in the day.